In electrical systems for controlling an entrance gate and/or a garage door of a dwelling, it is typical to locate the various units which make up the system where it is necessary, and connect them through electric wires.
In a dwelling, the following, for example, may be installed: a motor for one wing of the gate, a motor for the other wing of the gate, a motor for the garage door, a radio receiver for receiving the requests of movement of the gate and of the door coming from remote control transmitters, key-operated selectors and/or command keypads for operating the gate or the door, various safety photocell systems (each photocell system is composed of a receiver and a transmitter) or equivalent sensors, and luminous and/or acoustic devices for signalling the movement of the gate and the door.
Until a short time ago, the number of devices installed was fairly small (four or five) and these devices, for the sake of simplicity, were located very close to each other, for example next to the gate.
Recently, international safety regulations, on the one hand, and the requirements of users, on the other hand, have resulted in a considerable increase in the number of devices installed (often ten or so or even more) and in the impossibility of locating them close to each other; this tendency will continue in the future. Installed devices of this kind are electrically connected to the electrical control system; more precisely, generally these devices are connected directly and locally to the peripheral units of the electrical control system, which units are connected directly, by means of electric wires, to the central unit of the electrical control system, the “heart” of the system.
An important aspect of an electrical system for controlling a closing element which is moved electrically is safety. In particular, it is necessary to prevent the movement of the closing element from inadvertently causing damage to objects and, in particular, injury to persons.
A quite common way of obtaining this result is to identify an area inside which the movement of the closing element occurs and to stop this movement if an object or a person enters into this area. The safety devices most used for monitoring areas in these applications are photocell systems. As is well known, a photocell system is composed of a transmitter and a receiver. If the system is correctly installed, when the system is active, the transmitter emits light rays which are received by the receiver, and if an object or a person passes in between the transmitter and the receiver, the light rays do not reach the receiver and this situation is signalled by the system.
For proper monitoring of an area it is often necessary to use more than one photocell system and a suitable arrangement along the perimeter. FIG. 5 shows, in schematic form, a sliding gate CA located between two walls M1 and M2, two photocell systems FC1, FC2 composed respectively of two transmitters FC1-T, FC2-T and two receivers FC1-R, FC2-R.
Apparently the two photocell systems monitor adequately the area where the gate CA moves. If a person P, for example crosses the line which joins the transmitter FC2-T and the corresponding receiver FC2-R, the light beam of the photocell system FC2 is interrupted and the gate CA stops.
In reality, the two photocell systems FC1 and FC2 may interfere with each other. In fact, due to the closeness of the two systems FC1 and FC2, the light rays emitted by the transmitter of one of the two photocell systems reach the receiver of this photocell system, but also the receiver of the other photocell system (albeit with a reduced intensity). In this case, and as shown in FIG. 5, there is the risk that, even if the person P has entered into the monitored area, the gate CA is not stopped by the electrical control system because the two receivers FC1-R and FC2-R continue to receive light rays.